Three-frequency relaying system

ABSTRACT

A frequency shift relaying system utilizing at least one transmitter which is normally transmitting its signal at a first or normal frequency and operable to shift the frequency of its output signal to a second or a third frequency one of which is above and the other of which is below the normal or first frequency in response to a first or a second fault condition and utilizing at least one receiver means tuned to receive the transmitted signal at the first and second and third frequencies; the receiver means including a pair of output circuits operable in accordance with frequency of the transmitted signal, and effective to maintain a first of the output circuits in a first or a second operating condition in response to the transmission of the signal at the first or the second frequency and to maintain a second of the output circuits in a third or a fourth operating condition in response to the transmission of the signal at the first or third frequency.

United States Patent Ray et al.

[54] THREE-FREQUENCY RELAYING SYSTEM Inventors:

[72] Roger E. Ray, Parsippany, N..I.; Nicholas Perez-Stable, W. Palm Beach, Fla.

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

Mar. 19, 1970 Filed:

Appl. No.:

[56] References Cited UNITED STATES PATENTS 3,501,704 3/1970 Teitelbaum 1 78/ 66 3,443,159 5/1969 Altfather ..325/30 1 9A 4A BA lOA BREAKER TR AP V V V 3 D STANCE DIFFERENTIAL RELAY RELAY OR 24A GROUND 26A RELAY 102A THREE FREQUENCY TRANSMITTER THREE 19A FREgl J EIgg Feb. 15, 1972 FOREIGN PATENTS OR APPLICATIONS 673,330 10/1963 Canada ..325/ l 63 [57] ABSTRACT A frequency shift relaying system utilizing at least one transmitter which is normally transmitting its signal at a first or normal frequency and operable to shift the frequency of its output signal to a second or a third frequency one of which is above and the other of which is below the normal or first frequency in response to a first or a second fault condition and utilizing at least one receiver means tuned to receive the transmitted signal at the first and second and third frequencies; the receiver means including a pair of output circuits operable in accordance with frequency of the transmitted signal, and effective to maintain a first of the output circuits in a first or a second operating condition in response to the transmission of the signal at the first or the second frequency and to maintain a second of the output circuits in a third or a fourth operating condition in response to the transmission of the signal at the first or third frequency.

11 Claims, 6 Drawing Figures 4B 9B [0B 8B 2 r 28 g VA AA TRAP BREAKER i {I28 I68 FFERENTIAL DISTANCE A RELAY RELAY GROUND n2- RELAY -22e THREE FREQUENCY TRANSMITTER 20B] 27B ma BB 1 I THREE [9 l FREQUENCY r 1 RECEIVER AND PATENTEDFEB 15 I972 mmk zm FDAFDO KEE ESE mm om v SHEET u 0F 4 wa P BRIEF SUMMARY OF THE INVENTION Powerline carrier signal transmission is used for transmitting relay information from one line terminal to another. The band of frequencies available for transmission-is a band extending from 30 to 200 kilohertz. With the increased use of carrier transmission and the rapid growth of power systems, the finding of available frequencies for newlines in apower system is difficult. This invention provides for a more efficien use of the available carrier band or spectrum.

In the past, the unblocked relaying system and the direct transfer trip system used separate carrier channels for transmitting the required information. These channels of necessity were separated by a minimum of 1.5 kilohertz when transmitting in the same direction and a minimum of 3 kilohertz between the transmissions in the different directions.

In accordance with this invention a single transmitter is used for each direction. Each transmitter is capable of transmitting its signal atany of three frequencies. Normally during nonfault conditions it transmits at a center or normal frequency, f Under one fault condition it will transmit its signal at a frequency which is slightly above normal frequency for example fl.+200 hertz to provide a first tripping or unblock signal and under a second fault condition it will transmit its signal below normal frequency, for example fl.-200 hertz to provide a second tripping or transfer trip signal. .When primary and backup channels are provided in each direction, the total required channel width may be as low as 6 kilohertz when compared with a required prior art channel width of 8% kilohertz.

The receiving terminal is provided witha receiver capable of receiving the signal'of [11200 hertz and provides an output signal much like two independent receivers. One of the receiver output signals responds to the frequency range f;l- 200 hertz and the other receiver output signal responds to the frequency range f -200 hertz. The logic network energized by the two receivers are interconnected to provide a system operation compatible with that presently used. A spectrum saving of 30 percent and a cost advantage are achieved by this invention. I

BRIEF DESCRIPTION OF .TI-IE DRAWINGS For a better understanding of the invention reference may be had to the preferred embodiment, exemplary of the invention; shown in the accompanying drawings in which:

FIG. I is a block diagram illustrating the primary relaying protection of a two-station frequency shift powerline carrier relaying system representing one embodiment of the invention;'

FIG. 2 illustrates in block diagram form, a more detailed representation of the three-frequency receiver embodying the invention;

FIG. 3 illustrates the frequency relationships of the apparatus of FIGSJ and 2;

FIGS. 4A and 4B-'illustrate the difference in the spectrum required by the prior art and by the invention; and,

FIG. 5 is a schematic diagram of a transmitter circuit suitable to provide the signal at the three frequencies.

DETAILED DESCRIPTION Referring to the drawings by characters of reference, the numerals 1A and 18 represent a relaying apparatus located at the A and B ends of a transmission line 2 which interconnects a pair of buses 4A and 4B. The line 2 may operate at a relatively high voltage as for example 220 kilovolts and may be a conventional polyphase line although shown for simplicity in single-line arrangement. The line secton 2A connects the bus 4A to the transmission line 2 through a suitable breaker 8A and a frequency trap 10A. Similarly the line section 28 connects the bus 48 to thetransmission line 2 through a similar breaker 8B and trap 108. The relaying networks 1A and 1B are energized from the line portions 2A and 28 through suitable current and potential transformers diagrammatically shown. a

The relaying 7 network 1A includes a fault-responsive distance relay 12A and a. ground fault-responive relay 14A. The outputs of the relays 12A and 14A are connected to the two input terminals. of an OR-network 16A which has its output terminal connected to one input terminal 17A of an AND- network 18A. The other input terminal 19A of the AND-network 18A is connected to one output terminal 21A of a threefrequency receiver 20A. The other output terminal 27A of the receiver 20A and the output terminal of the AND-network 18A are each connected to the breaker 8A. In the event of simultaneous output signals from the OR-network 16A and the output 21A of the receiver 20A, the AND-network 18A will be energized and trip the breaker 8A. As will be explained in greater detail below, the output terminal 21A is energized as a consequence of the sensing of a fault by the distance relay 123 or by the ground relay 14B and the consequent change in the frequency of the output signal of the transmitter 223 changing from thefrequency f to the frequency f to transmit an unblock-frequency signal to the receiver 20A. Similarly the breaker 8B is tripped in response to the concurrent or simultaneous. application of the output signal of the OR-network 16B to the input terminal 178 and of the receiver 208 to the input terminal 19B of the AND-network 188.

In the event of a fault in the transformer 9A, a differential protecting relay 24A will energize its two output terminals 25A and 26A. Energization of the output terminal 25A causes the three frequency transmitter 22A to change the frequency of its transmitted signal from f to f to provide a transfer trip signal to the three-frequency receiver 208. With the receiver 208 energized at the f frequency it energizes it output terminal 27B whereby the breaker 8B is actuated to disconnect the line 2 and the transformer 9A from the bus 48, concurrently with the energization of the terminal 25A. The output terminal 26A of the differential relay trips the breaker 8A and the line section 2A and transformers 9A are disconnected from the bus 4A.

In FIG. 1 only a primary protection system is illustrated however, a backup protection system maybe used to provide secondary protection. FIG. 4A illustrates the required 6- kilohertz frequency spectrum with a 3-kilohertz spacing between the transmission in the two directions. FIG. 4B shows the A spectrum for similar protection in accordance with the present art to 8% kilohertz. The secondary protection network can take substantially thesame form as that illustrated in FIG. I for the'primary-protection network or other known forms and its further illustration in this application isb elieved urinecessary.

As illustrated in FIG. 2, the threefrequency receiver 20 comprises a first receiver portion 30 and a second portion 32. The receiver portion 30 energizes the receiver output terminal 21 while the receiver portion 32 energizes the output terminal 27. Referring first to the receiver portion 30, the input signal is supplied to its input terminal 34 through a wide-band filter 35 having a frequency characteristic 35, a mixer 36, a second filter 37 which may, for example, have a center frequency of 20 kilohertz and into a discriminator 38 which has an output characteristic 38. An oscillator 39 supplies a second frequency signal to the mixer 36 which second frequency may be that of the normal frequency (f plus 20.2 kilohertz. The discriminator 38 is of normal construction and (see curve 38") provides a first output signal at its output terminal 40 when the input signal fed to the discriminator is at a frequency of f no output to either of'its output terminals 40 and 41 when the inputsignal is at a frequency f}, hertz, and provides a second output signal at its output terminal 41 when the input signal is at a frequency off hertz.

The terminal 40. of the discriminator 38 is connected to the lower terminal 42 of the OR-network 43 and when energized applies a 1 signal to the input terminal of the OR network to supply a 1 signal to the timer 44, to the NOT-input terminal 45 of the AND-network 46, and to the input terminal 47 of an OR-network 48. Upon the timing out of the timer 44 (120 msec.) a 1 signal will be applied to the input terminal 49 of the OR-network 50. The oR network 50 will in turn apply a' l signal to the timer 52 which substantially instantaneously times out to apply a 1 signal to the terminal 53 of the AND- network 46, to the input terminal 54 of the AND-network 55, and to the NOT-input terminal 73 of the low-signal alarm 72. The NOT-input terminal 45 of the AND-network 46 reverses the l signal into a signal and the AND-network 46 will maintain a 0 signal at its output terminal 21 and thereby at the input terminal 1940f the AND-network 18. Under this condition the breaker 8 cannot be tripped irrespective of the operation of the relays l2 and 14.

vWhen the frequency of the received signal changes to the unblocked frequency (f the signal at output terminal 40 of the discriminator 38 and that of the OR-network terminal 42 becomes 0 and the signal at the output terminal 41 becomes lwithin a very small interval of time. As will be described in greater detail below the signal at the other input terminal 56 of the OR-network 43 will be 0 so that a 0 signal is supplied to the timer 44, to the terminal 47 of the OR-network 48, and to the NOT-terminal 45 of the AND-network 46. This results in the initiation of the timing out operation of the time 52 which, as indicated, may be 150 msec. During this period the timer will maintain the 1 signal at the terminal 53 of the AND-network 46. Since the OR-network 43 is now supplying a 0 signal to the NOT-input terminal 45, the AND-network 46 energizes its output terminal with a 1 signal. If the OR-network 16 is now supplying the AND-network 18 with a 1 signal, the breaker 8 will be tripped. v

The AND-network 55 is provided to reenergize the timer 52 to maintain the signal at its output terminal as long as the signal supplied to the receiver portion is at the f frequency. The terminal 54 will be supplied with a 1 signal for at least l50 msec. after the unblock frequency f occurs. Within a very short interval after the occurrence of the f frequency, the discriminator 38 will provide a 1 signal at its output terminal 41 and supply a Isignal to the input terminal 58 of the AND-network 55 and to the input terminal 57 of the OR-network 48 to replace the lsignal at the input terminal 47 which was lost when the 1 signal was removed from the terminal 42 of the OR-network 43. The resulting 0 signal from the NOT- output terminal 80 of the OR-network 48 permits the NOISE- network 78 to reset and supply a 0 signal to the NOT-input terminal 59 of the AND-network55 and causes thenetwork 55 to supply a 1 signal to the OR-network 50 which substantially immediately resets the timer 52 and will prevent the timing out of its 150 msec. interval and the AND-network 46 will maintain the 1 signal at the output terminal 21. i

The terminal 41 establishes a 1 signal at the input terminal 71 of the AND-network 76 and at the input terminal 57 of the OR-network 57 whereby an 0 signal will be maintained at the input terminal 79 of the NOISE-network and a 0 signal will be maintained thereby on the NOT-input terminal 84. The AND- network 76 will supply a 1 signal to the timer 74 which times out in a short interval, indicated as being 3 msec., to supply a 1 signal to the terminal 56' of the OR-network 43 in the receiver portion 32 for a purpose to be made clear below. The l signal output of the timer 74 is also supplied to unblock checkback indicator 86.

The NOlSE-network 78 can take the form of a timer which will integrate the time intervals that l signals are absent from both of the input terminals 47 and 57. Such an occurrence is caused by the discriminator as it alternates the signal condition at its output terminals 40 and 41 in response to the noise condition of the channel supplying the input signal to the receiver 20. the network 72 will also respond to a loss of signal or loss of channel which after the 150 msec. delay of the timer 52 will prevent a 1 signal from occurring at the terminal 21. A timer such as that illustrated in the Altfather US. Pat. No. 3,443,159 dated May 6', 1969 operates to provide this function.

The receiver portion 32 is quite similar to the receiver portion 30 and like logic elements in receiver portion 32 are designated by the same reference character except that the characters used in the receiver portion 30 are primed. Additionally, the receiver portion 32 omits the timer 74 but additionally has a timer 62 and an AND-network 64. The AND- network has an input terminal 65 connected to the output terminal 68 of the timer 62, an input terminal 66 connected to the output terminal 41 of the discriminator 38' and an output terminal 67 connected to an input terminal 69 of the AND- network 46. The timer 62 is provided with a NOT-input terminal 70 which is connected to the output terminal of the OR- network 43'.

During the periods in which a signal at the frequency f, is being supplied to the receiver 20, the discriminator 38' will provide a 1 signal at its output terminal 40' and a 0 signal at its output terminal 41' as indicated by the discriminator curve 38". The output terminal 40' is connected to the input terminal 42 of the network 43' and the output terminal 41' is connected to the terminal 57' of the OR-network 48', the terminal 71 of the AND-network 76, the terminal 58' of the AND-network 55' and the terminal 66 of the AND-network The output terminal of the OR-network 43' is connected to the input terminal of the timer 44, to the NOT-input terminal 70 of the timer 62, and to the input terminal 47' of the OR- network 48. When the timer 44 times out, it maintains a 1 signal at the input terminal 49' of the OR-network 50 whereby the OR-network 50' maintains a 1 signal to maintain a 1 signal at the input terminal 53 of the AND-network 46', at the input terminal 54' of the AND-network 55 and at the NOT-input terminal 73 of the loss of signal alarm 72'. The discriminator output terminal 41' is connected to the input terminal 58' of the AND-network 55' to the input terminal 57 of the OR-network 48', to the input terminal 71 of the AND- network 76 and to the input terminal 66 of the AND-network 64. With the signal at the f frequency being supplied to the receiver 20, the AND-network 64 maintains a 0 input to the input terminal 69 of the AND-network 46 whereby the AND- network 46' is prevented from supplying a 0 or trip signal to its output terminal 27 to cause a tripping of the breaker.

When, the signal supplied to the receiver is at the unblocked frequency f both output terminals 40 and 41 of the discriminator 38 have a 0 output signal. As long as the 0 output signal is maintained on the output terminal 41' the AND-network 46' cannot be actuated to falsely trip the breaker. The supplying of the signal at the unblocking frequency does however result in a 0 signal at the output terminal 40' of the discriminator 38 and at the input terminal 42 of the OR-network 43'. This, in the absence of a 1 signal at the terminal 56, would result in the actuation of the loss of signal alarm 72 and the noise alarm 82'. To prevent these false alarms, the terminal 56 is supplied with a l signal from the receiver portion 30 during intervals in which a signal at the unblock frequency is received by the receiver 20. Thus the OR-network 43' is maintained with a 1 signal at its output to maintain the receiver portion 32 in the same operating condition as existed with an input signal of the frequency 1",.

Assuming now that the transmitter is transmitting a signal at the transfer trip frequency f the tenninal 41' will have a 1 signal and the terminal 40' will have a 0 signal. With the discriminator terminal 40' maintaining a 0 signal, both of the OR-input terminals 42 and 56' will be energized with a 0 signal and the output terminal of the OR will supply a 0 signal to the timer 44' to the input terminal 47 of the OR-network 48' and to the NOT-input terminal 70 of the timer 62. The timer 44 substantially immediately times out to put a 0 signal on the terminal 49. This change in signal may occur prior to the occurrence of a 1 signal at the input terminal 60' due to the delay between the removal of the 1 signal at the terminal 40' and the establishment of the 1 signal at the terminal 41.

The establishing 'of the 1 signal at the output terminal 41 places a 1 signal on the input terminal 58 of the AND-network 55' which occurs prior to the timing out of the timer 52', so that the AND-network 55' is effective to place a 1 signal on the input terminal 60' to lock the timer 52' in a condition to place a 1 signal on the input terminal 53' of the AND-network 46'. The signal to the NOT-input terminal 70 of the timer 62 causes the timer 62 to substantially immediately establish a 1 signal at its output terminal 68 and at the input terminal 65 of the AND-network 64. The input terminal 66 is supplied with a 1 signal from the terminal 41. Since both of the input terminals 65 and 66 of the AND-network 64 have a l'signal, a 1 signal will be applied to the input terminal 69 of the AND-network 46'. The NOT input terminal 45' of the AND-network 46' is energized with a 0 signal in the absence of noise and the AND-network 46' is effective to supply a 1 signal to the output terminal 27' which trips the associated breaker 8.

The placing of a 1 signal on the terminal 41' also placed a 1 signal on the input terminal 71' of the AND-network 76 whereby in the absence of noise the AND-network 72' places a 1 signal on the input terminal 56 of the OR-network 43 to maintain the OR-network 43 in the same condition as it was when a signal at the frequency f, was applied to the input terminal 34. It will be apparent from the above description that this prevents the loss of signal indicator 72 from being actuated to give a false indication of loss of signal as well'as a false trip or 1 signal at the terminal 21.

The receiver portion 32 includes a noise-responsive network 78' which has its input terminal 79 connected to the NOT-output terminal 80' of the OR-network 48 in a manner similar to that described in connection with the discussion of receiver portion 30. In the absence of noise or loss of channel the NOISE-network 78 will maintain a 0 signal at the NOT- input terminals 45- and 84' so that the associated AND-network 76 and 46' are rendered effective to supply a 1 signal at their output terminals in response to the application of a 1 signal to the input terminals 53' and 69 of the AND-network 46 and input terminal 71 of the AND-network 76'. If however there should be a loss of signal or channel or undue noise should occur in the channel between the transmitter and receiver the output terminal'of the noise network 78 will be held in a 1 condition to prevent the AND-network 76' from establishing a 1 signal at its output terminal and consequently energization of the transfer trip alarm. The AND-network 64 will be preventedfrom establishing a 1 signal at the terminal 69 whereby the AND-network 46' is prevented from establishing a 1 signal at the output 27.

FIG. shows schematically, a circuit, which may be used for the three-frequency transmitter 22. The input lead 100 is connected to-the output terminal 25 of the associated differential relay while the input lead 102 is connected to the output'terminal of the associated OR-network 16. Normally the transfer trip oscillator 104 which includes the transistor Q52- and the crystal 105 is tuned to oscillate at a given frequency which for example may be 2.1 megahertz. The unblock oscillator 106 which'includes the transistor Q and the crystal 107 is tuned to a second frequency which for example may be 2.0 megahertz. The output frequency in this instance will be the beat frequency of the two oscillators or 100 kilohertz which is the f frequency.

When a 1 signal is applied to the lead 100, the transistor Q 2.

conducts to provide base drive for the transistor Q The conduction of transistor Q results in a small decrease in the output frequency of the oscillator 104 by 200 hertz. This reduces signal frequency transmitted by the transmitter by 200 hertz to provide the trip signal at the f frequency. Similarly when a 1 signal is supplied to the input lead 102, the transistor Q conducts to provide base drive for the transistor G The conduction of transistor Q decreases the frequency of the oscillator 106 by 200 hertz to provide the unblock signal at the f frequency. The beat or difference frequency is amplified by the transistors Q and 0 The transmitter in the event of l signals being applied at the same time to the leads 100 and 102 as might occur for simultaneously existing faults to give priority to one thereof. In the instance illustrated herein, the priority is given to the 1 signal on the lead which is the transfer trip signal having the frequency f For this purpose a diode D is connected between the common junction of the Zener diode Z and diode D in the base circuit of transistor Q22 and the common junction of the collector electrode of transistor Q and resistor R With the diode D polarized as shown, with its anode connected to the common junction, conduction of transistor Q12 effectively connects the base of transistor O to the negative bus and shunts the 1 signal which may be supplied to the lead 102. It will be quite apparent that by connecting the diode between the base circuit of the transistor Q and the collector of the transistor Q priority would be given to the signal at the frequency f Normally the transmitter provides a relatively low-power output signal. When, however, a 1 signal is received on either of the leads 100m 102, the resulting conduction of the transistor On or Q as the case may be, will result in the conduction of the transistor Q and a reduction of the emitter resistance of the buffer-amplifier transistor Q This power change may be in the order of 10 and shift from a l-watt output to a l0-watt output.

The signal is fed to the drive stage transistors Q and 0 from whence it is amplified to the 1- or l0-watt output power 108 as set forth and passed through the output filter l 10 to the coupling capacitors..l12 and 114 and one of the power lines to the receiver.

It will now be apparent that there has been disclosed a three frequency relaying system which will substantially decrease the amount of frequency spectrum required for relaying operations.

- What is claimed and is desired to be secured by United States Letters Patent is as follows:

1. In an intelligence exchanging system, a transmitter adapted to transmit an output signal at a normal frequency and at first and second actuated frequencies, said first and second actuated frequencies being of different magnitudes with respect to said normal frequency, means normally maintaining said transmitter in condition to transmit said signal at said normal frequency, first intelligence-responsive actuating means connected to said transmitter and effective when actuated to cause said transmitter to transmit said signal at said first actuated frequency, second intelligence-responsive actuating means connected to said transmitter and effective when actuated to cause said transmitter to transmit said signal at saidsecond actuated frequency, receiving apparatus operable to receive said signal at said first and second actuated frequencies and at said normal frequency, said receiving apparatus including first and second output means and discriminator means controlling the actuation of said output means, said output means each having a normal and. an actuated operating condition, first timing means effective when energized to time-out andestablish a first timing interval subsequent tothe energization thereof, said discriminator means being effective when signal is at said normal frequency to maintain each of said output means in its normal condition and to energize said, timing means, said discriminator means being effective when said signal is at said first actuated frequency and solely subsequent to the timing out of said first timing interval toplace said first output means in its said actuated condition, saiddiscriminator means being effective when said signal is at said second actuated frequency to place said second output means in its said actuated. condition, said receiving apparatus including control means actuated as a consequence of change in the frequency of said signal to said second frequency to maintain said timing means in its said timed-out condition.

2. The combination of claim 1 in which said actuated frequencies are of opposite magnitude with respect to said normal frequency and inwhich filter means is provided to exclude frequencies which lie outside of the frequency range between said actuated frequencies.

3. The combination of claim 2 in which said actuated frequencies differ from said normal frequency be equal arithmetic magnitudes and in which said filter means prevents said discriminator means from actuating said timing means when said signal frequency is substantially beyond the range of said normal and said first actuated frequency.

4. The combination of claim 1 in which said receiver is provided with a second timing means effective to time out and establish a second timing interval subsequent to the energization thereof, said discriminator means being effective when said signal is at its said second actuated frequency and solely subsequent to the timing out of said second timing interval to place said second output means in its said actuated condition, and control means actuated as a consequence of a change in the frequency of said signal to said first actuated frequency to maintain said second timing means in its said timed-out condition.

5. The combination of claim 1 in which said transmitter includes means to rendersaid signal at said first actuated frequency whenever both of said intelligence-responsive actuated means are actuated whereby said first output means is placed in its said actuated condition as a consequence of the said first signal at said selected frequency. I

6. Receiving apparatus for receiving an input signal at normal and first and second actuated frequencies, each of said actuated frequencies being different from each other and from said normal frequency, said apparatus comprising discriminator means responsive to the frequency of said signal to provide any of four possible output signals depending upon the frequency of said input signal, a setting device having an initial and a set condition, said setting device being transferred from its said initial to its said set condition in response to the existence of a first of said four output signals, first circuit means including said setting device and actuated by said discriminator from a first to a second operating condition in response to the existence of a second of said four output signals and the existence of said set condition of said setting device, second circuit means energized into a first or a second operating condition by said discriminator means in response to the sequential existence of a third or a fourth of said four output signals respectively, and a. network interconnecting said first and second circuit means and effective to maintain said setting device in its said set condition in response to the rendering of said second circuit means in its said second operating condition. I

7. The combination of claim 6 in which said network includes a timer effective to place said one network in its said first operating condition solely subsequent to a predetermined time interval subsequent to the return of said signal to its said normal frequency.

8. Receiving apparatus for receiving an input signal at a normal frequency and at first and second actuated frequencies, each of said actuated frequencies being different from each other and from said normal frequency, said apparatus comprising, an input signal receiving means, first and second dis criminator networks connected thereto for energization therefrom, each said network having first and second output terminals, each said network being effective to provide a first energization of its said output terminals when an input signal of said normal frequency is being supplied to said signalreceiving means, said first discriminator being effective to provide a second energization of its said output terminals when an input signal of said first frequency is being supplied to said signal-receiving means, said second discriminator being effective to provide a third energization of its said output terminals when an input signal of said second frequency is being supplied to said signal-receiving means, first and second tripping networks, each said tripping network having a first timing device operable when actuated from an initial condition to time out a desired time interval, each said tripping network having an AND network, each said AND network having first and second input terminals and an output terminal which provides an output si nal when first signals are bein supplied concurrently to eac of its said input terminals, eac said tripcontrolling network having a first circuit including its associated said timing device connected to said first input terminal of its associated said AND network and a second circuit connected to said second input terminal of its associated said AND network, each said first circuit being effective to supply said first signal to said first input terminal of its associated said AND network solely when its associated said timing device is timed out, means individually connecting said first circuit of each of said first and said second tripping networks to said first output terminals of said first and second discriminators respectively, and connecting said second circuit of each said first and said second tripping networks to said second output terminals of said first and second discriminators respectively,

each said timing device being effective to time out and supply said first signal to said first input terminal of its associated said AND network subsequent to the reception of said signal at said normal frequency by said signal-receiving means for said desired time interval, said second circuits of said first and second tripping networks being effective to supply said first signal to said second terminal of its associated said AND network when an input signal at said first and second frequencies respectively, is being supplied to said signal receiving means, a third circuit interconnecting said tripping networks, said third circuit being energized by said second discriminator as a consequence of the energization of its said output terminals at said third energization and effective to maintain said first timing device of said first tripping network in its said timed out condition irrespective of the continued energization of said output terminals of said first discriminator at said first energization, and a fourth circuit interconnecting said tripping networks, said fourth circuit being energized by said first discriminator as a consequence of the energization of its said output terminal at said second relative energization and effective to maintain said first timing device of said second tripping network in its said timed-out condition irrespective 'of the continued energization of said output terminals of said second discriminator at said first relative polarity.

9. The combination of claim 8 in which each of said first circuits includes a second timing device intermediate its said first timing device and its associated said AND Network, each of said first devices being effective when deenergized to reset itself into its said initial condition without substantial time delay, each of said second timing devices having an initial condition and an actuated condition, each of said second timing devices being operable when actuated to change from its said initial condition to its said actuated condition without substantial time delay and to return to its said initial condition with substantial time delay after its said actuation has been removed.

10. The combination of claim 8 in which said signal-receiving means includes filter means which substantially attenuates frequencies outside of a first range of frequencies which includes said normal and said actuated frequencies, and to supply said first range of frequencies to one of said discriminators.

11. The combination of claim 10 in which said filter means attenuates frequencies outside of a second range of frequencies which includes solely said normal and one of said actuated frequencies and to supply said second range to the other of said discriminators. 

1. In an intelligence exchanging system, a transmitter adapted to transmit an output signal at a normal frequency and at first and second actuated frequencies, said first and second actuated frequencies being of different magnitudes with respect to said normal frequency, means normally maintaining said transmitter in condition to transmit said signal at said normal frequency, first intelligence-responsive actuating means connected to said transmitter and effective when actuated to cause said transmitter to transmit said signal at said first actuated frequency, second intelligence-responsive actuating means connected to said transmitter and effective when actuated to cause said transmitter to transmit said signal at said second actuated frequency, receiving apparatus operable to receive said signal at said first and second actuated frequencies and at said normal frequency, said receiving apparatus including first and second output means and discriminator means controlling the actuation of said output means, said output means each having a normal and an actuated operating condition, first timing means effective when energized to time out and establish a first timing interval subsequent to the energization thereof, said discriminator means being effective when signal is at said normal frequency to maintain each of said output means in its normal condition and to energize said timing means, said discriminator means being effective when said signal is at said first actuated frequency and solely subsequent to the timing out of said first timing interval to place said first output means in its said actuated condition, said discriminator means being effective when said signal is at said second actuated frequency to place said second output means in its said actuated condition, said receiving apparatus including control means actuated as a consequence of change in the frequency of said signal to said second frequency to maintain said timing means in its said timed-out condition.
 2. The combination of claim 1 in which said actuated frequencies are of opposite magnitude with respect to said normal frequency and in which filter means is provided to exclude frequencies which lie outside of the frequency range between said actuated frequencies.
 3. The combination of claim 2 in which said actuated frequencies differ from said normal frequency be equal arithmetic magnitudes and in which said filter means prevents said discriminator means from actuating said timing means when said signal frequency is substantially beyond the range of said normal and said first actuated frequency.
 4. The combination of claim 1 in which said receiver is provided with a second timing means effective to time out and establish a second timing interval subsequent to the energization thereof, said discriminator means being effective when said signal is at its said second actuated frequency and solely subsequent to the timing out of said second timing interval to place said second output means in its said actuated condition, and control means actuated as a consequence of a change in the frequency of said signal to said first actuated frequency to maintain said second timing means in its said timed-out condition.
 5. The combination of claim 1 in which said transmitter includes means to render said signal at said first actuated frequency whenever both of said intelligence-responsive actuated means are actuated whereby said first output means is placed in its said actuated condition as a consequence of the said first signal at said selected frequency.
 6. Receiving apparatus for receiving an input signal at normal and first and second actuated frequencies, each of said actuated frequencies being different from each other and from said normal frequency, said apparatus comprising discriminator means responsive to the frequency of said signal to provide any of four possible output signals depending upon the frequency of said input signal, a setting device having an initial and a set condition, said setting device being transferred from its said initial to its said set condition in response to the existence of a first of said four output signals, first circuit means including said setting device and actuated by said discriminator from a first to a second operating condition in response to the existence of a second of said four output signals and the existence of said set condition of said setting device, second circuit means energized into a first or a second operating condition by said discriminator means in response to the sequential existence of a third or a fourth of said four output signals respectively, and a network interconnecting said first and second circuit means and effective to maintain said setting device in its said set condition in response to the rendering of said second circuit means in its said second operating condition.
 7. The combination of claim 6 in which said network includes a timer effective to place said one network in its said first operating condition solely subsequent to a predetermined time interval subsequent to the return of said signal to its said normal frequency.
 8. Receiving apparatus for receiving an input signal at a normal frequency and at first and second actuated frequencies, each of said actuated frequencies being different from each other and from said normal frequency, said apparatus comprising, an input signal receiving means, first and second discriminator networks connected thereto for energization therefrom, each said network havinG first and second output terminals, each said network being effective to provide a first energization of its said output terminals when an input signal of said normal frequency is being supplied to said signal-receiving means, said first discriminator being effective to provide a second energization of its said output terminals when an input signal of said first frequency is being supplied to said signal-receiving means, said second discriminator being effective to provide a third energization of its said output terminals when an input signal of said second frequency is being supplied to said signal-receiving means, first and second tripping networks, each said tripping network having a first timing device operable when actuated from an initial condition to time out a desired time interval, each said tripping network having an AND network, each said AND network having first and second input terminals and an output terminal which provides an output signal when first signals are being supplied concurrently to each of its said input terminals, each said trip-controlling network having a first circuit including its associated said timing device connected to said first input terminal of its associated said AND network and a second circuit connected to said second input terminal of its associated said AND network, each said first circuit being effective to supply said first signal to said first input terminal of its associated said AND network solely when its associated said timing device is timed out, means individually connecting said first circuit of each of said first and said second tripping networks to said first output terminals of said first and second discriminators respectively, and connecting said second circuit of each said first and said second tripping networks to said second output terminals of said first and second discriminators respectively, each said timing device being effective to time out and supply said first signal to said first input terminal of its associated said AND network subsequent to the reception of said signal at said normal frequency by said signal-receiving means for said desired time interval, said second circuits of said first and second tripping networks being effective to supply said first signal to said second terminal of its associated said AND network when an input signal at said first and second frequencies respectively, is being supplied to said signal receiving means, a third circuit interconnecting said tripping networks, said third circuit being energized by said second discriminator as a consequence of the energization of its said output terminals at said third energization and effective to maintain said first timing device of said first tripping network in its said timed out condition irrespective of the continued energization of said output terminals of said first discriminator at said first energization, and a fourth circuit interconnecting said tripping networks, said fourth circuit being energized by said first discriminator as a consequence of the energization of its said output terminal at said second relative energization and effective to maintain said first timing device of said second tripping network in its said timed-out condition irrespective of the continued energization of said output terminals of said second discriminator at said first relative polarity.
 9. The combination of claim 8 in which each of said first circuits includes a second timing device intermediate its said first timing device and its associated said AND Network, each of said first devices being effective when deenergized to reset itself into its said initial condition without substantial time delay, each of said second timing devices having an initial condition and an actuated condition, each of said second timing devices being operable when actuated to change from its said initial condition to its said actuated condition without substantial time delay and to return to its said initial condition with substantial time delay after its said actuation has been removed.
 10. The combination of claim 8 in which said signal-receiving means includes filter means which substantially attenuates frequencies outside of a first range of frequencies which includes said normal and said actuated frequencies, and to supply said first range of frequencies to one of said discriminators.
 11. The combination of claim 10 in which said filter means attenuates frequencies outside of a second range of frequencies which includes solely said normal and one of said actuated frequencies and to supply said second range to the other of said discriminators. 